The invention is generally related to lasers and fiber optic communication links. More specifically, the invention is generally related to monitoring the performance and operational status of a laser and thereby maintaining the integrity of a fiber optic link.
Today, optical fibers form the backbone of a global telecommunication system. These strands of glass, each thinner than a human hair, are designed to carry huge amounts of data transmitted by tightly focused laser beams. Together, optical fibers and lasers have dramatically increased the capacity of the telephone and data systems. With equal improvements in computing, mankind has become dependant on this communication technology.
Thus, maintaining the integrity of a fiber optic communications link has become critical, particularly in these high capacity telephone and data systems. Failure to properly maintain a link can result in severed communications as well as data disruptions since both voice and data may be carried on the same fiber. This can lead to lost revenues since people often do not reestablish calls once they have been interrupted or xe2x80x9cdropped.xe2x80x9d Further, reestablished calls and data links must be rerouted by the host system over another link. This results in delays as well as the additional time and effort required for rerouting, not to mention reduced system capacity.
Often, breaks in these links can be traced to failures of laser diodes that transmit data over these fibers. These laser diodes function to transmit voice and data through modulation of their photonic emission. Laser diodes are xe2x80x9cbiasedxe2x80x9d by a DC current which causes them to emit radiation at a particular frequency. This emission frequency is then varied, or xe2x80x9cmodulated,xe2x80x9d by an AC current in response to voice or data which is desired to be transmitted. Experience has shown that most laser diodes do not fail catastrophically but rather slowly deteriorate in performance, drawing more and more bias and modulation current to generate the amount of output power necessary to maintain the link. At some point, the amount of current required becomes so burdensome to the electronic driver that the link is broken.
Once a link breaks, a technician must be dispatched to diagnose and repair the broken link. To diagnose the link, the technician must disconnect the optical fiber from the laser module and connect it to an optical power meter in order to measure the optical output power. Based on the optical output power, a diagnosis is made. Many times, the optical output power is too low due to a failure of the laser diode in the laser module. The technician must then either replace the laser diode or install another laser module. Often a new laser module is installed in the interest of time and cost, and the link is restored.
This process of measuring the optical output power of a laser can also be preformed in an effort to predict potential failures in the future. However, the skill of the technicians which perform the measurements becomes paramount as the orientation of the fiber in relation to the optical power meter can significantly effect the amount of output power measured. This along with the amount of time, effort, and manpower, as well as reduced system capacity, that accompany this approach make service providers which use fiber optic links reluctant to use this process.
In most systems, a laser diode controller is used to maintain constant laser output power. In these types of systems, a monitor photo-diode is integrated into the laser diode package and receives light from either the back facet of the laser diode or a back reflection of the main beam from the laser diode package window. Thus, the collected laser power incident on the monitor photo-diode is proportional to the outgoing optical power. The monitor diode then converts this representative average optical power to an average current which serves as a feedback signal to the controller. The controller then compares this feedback signal against a reference current in order to maintain constant output power. As such, these laser diode controllers provide no externally readable monitor feature which describes the laser power being produced. Rather, the only indicator of laser output power is used as a control signal for purposes of maintaining constant output power.
Further, many of these systems provide no means of externally monitoring the bias or modulation currents and thus, provide no manner of monitoring the health of such links. Other systems, on the other hand, have attempted to provide more robust control and monitoring functionality in a link itself. For example, some designs rely on microprocessor control and painstakingly thermally characterize each laser used in every laser module produced in an effort to preempt laser failure. From this thermal characterization, a table is created for each module that relates the bias and modulation currents to temperature through the operating range of the laser in the module. The table is then stored in memory provided in the laser module. The microprocessor in the laser module then includes firmware that routinely monitors the laser""s thermal operating condition as well as onboard digital-to-analog convertors that control and adjusted the bias and modulation currents. The microprocessor is then operative to compare the set points of the digital-to-analog converters with the bias and modulation currents stored in the memory-based look up table at the present operating temperature as a method of predicting laser failure. Obviously, the use of a microprocessor to provide more robust control and monitoring functionality is an expensive solution particularly when a host system may rely on multiple laser modules.
Therefore, a significant need exists in the art for a manner of cost effectively monitoring the output power of a laser in a fiber optical link or module, and in particular a manner of monitoring the output power of a laser without having to break a link.
The invention addresses these and other problems associated with the prior art by providing an apparatus, circuit arrangement, and method to monitor the output power and/or the bias of a laser used in a fiber optic link, which often does not require breaking the link. Output power is monitored in part through the use of a current mirror that outputs a signal proportional to the optical output power of the laser. By providing a signal which is proportional to optical power, a host system such as a computer or other data processing system has the ability to read the signal and calculate, as desired, the optical power output of the laser. Thereby, the host system is often further able to determine laser performance and preempt laser failure.
Similarly, the bias of a laser is monitored through the use of a current mirror that outputs another signal proportional to the bias of the laser. By providing a signal which is proportional to the bias, a host system has the ability to read this second signal and calculate the bias of the laser. Again, the host system is often able to use either one or both of these monitors to determine laser performance and preempt laser failure.
In one embodiment consistent with the invention, the signal that is proportional to the optical power is provided by current mirroring the monitor diode current typically used for maintaining constant output power. The mirrored current is then converted to a monitor voltage by a current to voltage converter circuit. In one embodiment of the invention, the monitor voltage is presented directly to an analog-to-digital converter within the host system. In another embodiment of the invention, the monitor voltage is digitized in a laser transceiver and delivered to the host system via a digital interconnect. In some embodiments, the host system may then be operative, using coefficients located in the transceiver nonvolatile memory, to calculate the optical power by reading the monitor voltage. As such, the host system is capable of monitoring the laser optical power output without disrupting the link.
In yet another embodiment consistent with the invention, the laser diode bias current is current mirrored. Once again, the mirrored current is converted to a voltage. This voltage, in one form or another, is monitored by a host system. The host system is then operative, using coefficients located in the transceiver module memory, to calculate the laser diode bias current.
These and other advantages and features, which characterize the invention, are set forth in the claims annexed hereto and forming a further part hereof. However, for a better understanding of the invention, and of the advantages and objectives attained through its use, reference should be made to the Drawings, and to the accompanying descriptive matter, in which there are described exemplary embodiments of the invention.